Typically, at some step in the manufacture of semiconductors, nitride is deposited on semiconductor wafers. A typical process for nitride deposition on semiconductor wafers of silicon includes reacting ammonia (NH.sub.3) with diclorosilane (SiH.sub.2 CL.sub.2, hereinafter referred to as DCS) to obtain silicon nitride (Si.sub.3 N.sub.4). The reaction is as follows: EQU 3SiH.sub.2 Cl.sub.2 +10NH.sub.3 .fwdarw.Si.sub.3 N.sub.4 +6NH.sub.4 CL+6H.sub.2
The NH.sub.4 CL and H.sub.2 are just by-products of the reaction. Typically, wafers are inserted inside a tube which has quartz (SiO.sub.2) walls for receiving silicon nitride at a low pressure. DCS and ammonia are then introduced and form the silicon nitride which is deposited upon the wafers. When a tube has been used for this purpose, a problem develops wherein the wafers which subsequently receive a nitride deposition have what appear to be streaks across their faces. Observation under a microscope reveals that the streaks are comprised of many spots of excessive nitride growth. Wafers which have such spots of excessive nitride growth are rejected because of the resulting non-uniformity on the surface which can cause problems with subsequent steps in the process. Attempts to solve the problem included varying tube pressure, varying tube temperature, varying DCS to ammonia ratio, introducing stand alone DCS and ammonia flows, and applying HCL. None of these, however, resulted in improvement of any significance.
One technique which did result in substantial improvement is to purge the quartz tube with oxygen between wafer runs. This technique is described in U.S. patent application Ser. No. 378,927, Hogan et al., entitled "Process For Improving Nitride Deposition On A Semiconductor Wafer", filed May 14, 1982, and assigned to the assignee hereof. At least one disadvantage of this technique is that there still remained a relatively high particle defect density.